Amorphous silica particles having a double structure, process for producing the same and use

ABSTRACT

The present invention provides the amorphous silica particles having a double structure of a core of the dense amorphous silica and a shell of the bulky amorphous silica, having a volume-based median diameter of from 1 to 5 μm and containing not more than 10% by volume of fine particles having a particle diameter of not larger than 0.5 μm. The amorphous silica particles of the double structure have constant particle diameters not only in the powdery state but also in a state where they are blended in a coating material or in a resin, without developing fine particles, exhibit excellent delustering action and antiblocking action, exhibit decreased abrading property, and exhibit excellent resistance against scars or abrasion.

BACKGROUND OF THE INVENTION Technical Field of the Invention

The present invention relates to amorphous silica particles having adouble structure, to a process for preparing the same and to the usethereof. More specifically, the invention relates to amorphous silicaparticles having a double structure of a core of the dense amorphoussilica and a shell of the bulky amorphous silica, to a process forpreparing the same, and to the use such as delustering agent for thecoating materials.

Prior Art

Coating materials have heretofore been blended with a fine silica powderas a delustering agent in order to lower the luster on the surfaces ofthe coated films. The delustering agent forms fine ruggedness on thesurfaces of the coated film to thereby lower the gloss value.

As delustering silica, there has been used a wet-method silica, i.e., awet-method silica obtained by neutralizing the sodium silicate with anacid. For example, Japanese Patent Publication No. 6669/1980 discloses adelustering composition comprising a fine powdery silica having arefractive index of larger than about 1.448 and an ignition loss ofsmaller than 2%, that is obtained by treating the ordinary wet-methodsilica with heat at a temperature of not lower than about 800° C.

Japanese Laid-Open Patent Publication No. 298014/1989 discloses aprocess for producing a delustering agent for coating materials,obtained by pulverizing wet-method silica until its average particlediameter becomes from 1 to 20 μm followed by the heat treatment at atemperature of from 400 to 1000° C. for 30 to 120 minutes.

Japanese Laid-Open Patent Publication No. 289670/1990 discloses adelustering agent comprising an inorganic hydrogel having a porousvolume of at least 1.0 ml/g, an average particle size of from 1 to 10microns, and a specific particle size distribution of when it isdispersed in a coating vehicle.

Furthermore, Japanese Laid-Open Patent Publication No. 117548/1993discloses a delustered coating composition containing a fine silicapowder and resin particles having a positive ζ-potential and an averageparticle diameter of from 0.01 to 5 μm.

Moreover, Japanese Laid-Open Patent Publication No. 166091/1995discloses a delustering agent for coating materials obtained by treatingthe surfaces of the wet-method precipitation silica with a polyethylenewax, and having a BET specific surface area of from 30 to 150 m² /g, anoil-absorbing amount of from 120 to 280 ml/100 g, a bulk density of from30 to 150 g/l, and an average particle diameter of secondary particlesof from 1.0 to 5.0 μm, the amount of the polyethylene wax being from 4to 12 parts by weight per 100 parts by weight of the wet-methodprecipitation silica.

The fine silica powder that has heretofore been used as the delusteringagent comprises shapeless secondary particles which are formed by theaggregation of primary particles having fine particle diameters.Therefore, the particle shapes are indefinite, particle sizes are widelydistributed and the particles are asymmetric. In a state of beingdispersed in a coating material, furthermore, it is difficult to controlthe diameter of the particles that are dispersed.

That is, a predetermined limitation is imposed on the diameter of thesilica particles that work to effectively lower the luster by formingruggedness on the surface of the coated film. The particles smaller thanthe effective particle diameter are not effective in decreasing theluster on the surface of the coated film, and the particles larger thanthe effective particle diameter form lumps on the surface of the coatedfilm to deteriorate the appearance and surface properties of the coatedfilm. Besides, in the conventional silica-type delustering agent, thecontent of the particles larger than the above-mentioned effectiveparticle diameter can be decreased by pulverization or classificationaccompanied, however, by an increase in the content of the particleshaving diameters smaller than the effective particle diameter, causinginconvenience to take place as described below.

First, the content of the particles having the effective particlediameters helpful for decreasing the luster on the surface is small and,hence, the amount of blending the silica-type delustering agent must beincreased per the coating material, requiring an increased cost andcumbersome blending operation. Moreover, with the particles havingdiameters smaller than the effective particle diameter being containedin the coating material, the viscosity of the coating material increasescausing the efficiency of coating operation to decrease. Besides, thepresence of particles of small diameters in the coating material causesthe hue of the coated film to become dark and dull, and further causesmechanical properties such as toughness of the coated film to bedeteriorated. Furthermore, the coated film containing aggregatedparticles of small diameters tends to be scarred on the surface due tofriction.

The conventional shapeless silica-type delustering agent may have aparticle constitution that is satisfactory to some extent before beingblended. When dispersed in the coating material, however, theconventional shapeless silica-type delustering agent tends to becollapsed (disintegrated) due to the shearing force, and are not capableof avoiding the above-mentioned defects.

Amorphous silica has another defect in that it causes abrasion and givesscars; i.e., the device for handling the coating material is abraded,and the coating material gets scarred when it is abraded. The slippingproperty can be improved when the surfaces of the amorphous silicaparticles are treated with a polyethylene wax or the like as proposed inthe foregoing without, however, radically solving the above-mentionedproblems.

Such defects of the amorphous silica are not confined simply in thedelustering agent for coating materials but also occur similarly evenwhen the amorphous silica is used as an antiblocking agent for the resinfilms.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provideamorphous silica particles having a novel structure free from theabove-mentioned problems, maintaining constant particle diameters notonly in the powdery state but also in a state where they are blended ina coating material or in a resin, without developing fine particles,exhibiting excellent delustering action and antiblocking action,exhibiting decreased abrading property, and exhibiting excellentresistance against scars or abrasion.

Another object of the present invention is to provide a method capableof easily preparing the amorphous silica particles having theabove-mentioned novel structure maintaining excellent productivity andeconomically.

A further object of the present invention is concerned to the use of theamorphous silica particles having the above-mentioned novel structureand, particularly, to provide a delustering agent for coating materialsand an antiblocking agent for films.

According to the present invention, there are provided amorphous silicaparticles having a double structure of a core of the dense amorphoussilica and a shell of the bulky amorphous silica, having a volume-basedmedian diameter (average particle diameter) of from 1 to 5 μm andcontaining not more than 10% by volume of fine particles having aparticle diameter of not larger than 0.5 μm.

In the amorphous silica particles of the present invention, it isdesired that:

1. a BET specific surface area is from 150 to 400 m² /g and a BET porousvolume is from 0.2 to 2 ml/g;

2. the cores of the dense amorphous silica and the shells of the bulkyamorphous silica are contained at a weight ratio of from 2:8 to 7:3 and,particularly, from 3:7 to 5:5;

3. the cores of the dense amorphous silica are gel-method amorphoussilica particles, and the shells of the bulky amorphous silica areprecipitation-method amorphous silica particles precipitated on thesurfaces of said core particles;

4. the cores of the dense amorphous silica has a BET specific surfacearea of from 200 to 800 m² /g, a BET porous volume of from 0.2 to 2ml/g, and a porous volume distribution in which a porous volume of poreshaving radii of from 10 to 150 angstroms is from 40 to 80% of the wholeporous volume.

According to the present invention, furthermore, there is provided aprocess for preparing amorphous silica particles having a doublestructure comprising:

a step of neutralizing an alkali silicate aqueous solution and a mineralacid aqueous solution under a condition of a pH of from 2 to 10 toprepare gel-method amorphous silica;

a step of wet-pulverizing the thus formed gel-method amorphous silica;and

step of neutralizing the alkali silicate aqueous solution and themineral acid aqueous solution in the presence of the wet-pulverizedgel-method amorphous silica particles under a condition of a pH of from5 to 9, so that the precipitation-method amorphous silica particles areprecipitated on the surfaces of the gel-method amorphous silicaparticles.

In the process of the present invention, it is desired that:

1. the gel-method amorphous silica particles are prepared at atemperature of not higher than 50° C.;

2. the gel-method amorphous silica is wet-pulverized so that thevolume-based median diameter becomes from 0.5 to 3 μm; and

3. the precipitation-method amorphous silica particles are precipitatedat a temperature of from 60 to 100° C.

According to the present invention, furthermore, there are provided adelustering agent for coating materials and an antiblocking agent forfilms, comprising the above-mentioned amorphous silica particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transmission-type electron microphotograph (magnification of30,000 times) of an amorphous silica particle having a double structureaccording to Example 1 of the present invention;

FIG. 2 is a transmission-type electron microphotograph (magnification of30,000 times) of gel-method amorphous silica particles; and

FIG. 3 is a transmission-type electron microphotograph (magnification of30,000 times) of precipitation-method amorphous silica particles.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[Amorphous Silica Particles of a Double Structure]

(1) The amorphous silica particles of the present invention have afeature in that they have a double structure of a core of the denseamorphous silica and a shell of the bulky amorphous silica. The doublestructure is confirmed upon observing the amorphous silica particlesusing a transmission-type electron microscope.

(2) FIG. 1 in the attached drawings is a transmission-type electronmicrophotograph of an amorphous silica particle having a doublestructure according to the present invention, FIG. 2 is atransmission-type electron microphotograph of gel-method amorphoussilica particles for comparison, and FIG. 3 is a transmission-typeelectron microphotograph of precipitation-method amorphous silicaparticles for comparison. It will be understood from this comparisonthat the amorphous silica particle having a double structure of thepresent invention includes a core of the dense amorphous silica at thecenter and has, in the periphery thereof, a covering (shell) of thebulky amorphous silica that appears like a floss.

(3) In the present invention, the dense amorphous silica works toenhance the strength of the particles, to prevent the particles fromcollapsing or from being powdered and, besides, works to improverefractive index of the particles to enhance transparency. On the otherhand, the bulky amorphous silica exhibits delustering action,antiblocking action, and improves resistance against abrasion and scars.

(4) However, when a coating material or a resin is blended with thedense amorphous silica particles, neither the delustering action nor theantiblocking action is exhibited to a satisfactory degree. Besides, theabrading tendency becomes conspicuous, and the film or the coating tendsto be scarred by abrasion to a conspicuous degree (see ComparativeExample 2 appearing later). When the coating material or the resin isblended with the bulky amorphous silica, on the other hand, thedelustering action and the antiblocking action are exhibited to someextent. However, the coating material and the resin tend to exhibit anincreased viscosity, and the transparency is deteriorated (seeComparative Example 3 appearing later). This is attributed to that thebulky amorphous silica particles collapse (disintegrate) the particlesto decrease the ratio of the particles having diameters that effectivelywork for exhibiting delustering action and antiblocking action, and thatfine particles formed by the collapse increase the viscosity. Theabove-mentioned tendencies are recognized even when the dense amorphoussilica and the bulky amorphous silica are simply mixed together (seeComparative Example 4).

(5) On the other hand, when the amorphous silica particles have a doublestructure by using the dense amorphous silica as cores and the bulkyamorphous silica as shells, 1 the particle diameter remains constant notonly in the powdery state but also in the state of being blended in thecoating material or in the resin, without forming fine particles, 2excellent delustering action and antiblocking action are exhibited, theabrading tendency is exhibited little, and excellent resistance isexhibited against scars and abrasion, and 3 the coating material or theresin composition blended with amorphous silica particles exhibitsexcellent transparency and deep appearance (see Example 1).

(6) The reasons are attributed to that the shells of the bulky amorphoussilica work to enhance the delustering action and antiblocking action,to decrease the abrasion tendency and to improve resistance againstbeing scarred, and the cores of the dense amorphous silica work toprevent the particles from being collapsed or becoming powdery, andmaintain the size of the amorphous silica particles in a range which ismost suited for exhibiting the delustering action and antiblockingaction, suppressing an increase in the viscosity caused by a powder andcontributing to improving the transparency.

(7) In the amorphous silica particles of the present invention, it isimportant that the volume-based median diameter is from 1 to 5 μm andthe content of fine particles having particle diameters of not largerthan 0.5 μm is not larger than 10% by volume. This is because, with themedian diameter lying within the above-mentioned range, the delusteringaction and antiblocking action are exhibited to a maximum degree with asmall blending amount. By suppressing the content of fine particles tobe not larger than the above-mentioned range, furthermore, it is allowedto suppress the viscosity of the blended coating material or resin.

(8) In relation to the above-mentioned double structure, the amorphoussilica particles of the present invention have a BET specific surfacearea of from 150 to 400 m² /g and a BET porous volume of from 0.2 to 2ml/g. When the specific surface area is smeller than the above-mentionedrange, the transparency tends to decrease. When the specific surfacearea is larger than the above-mentioned range, on the other hand, thestrength of the particles tends to decrease. When the porous volume isgreater than the above-mentioned range, the strength of the particlestends to decrease. When the porous volume is smaller than theabove-mentioned range, on the other hand, the abrasion tends toincrease.

(9) In the amorphous silica particles of the present invention, it isdesired that the cores of the dense amorphous silica and the shells ofthe bulky amorphous silica exist at a weight ratio of from 2:8 to 7:3and, particularly, from 3:7 to 5:5 from the standpoint of maintainingbalance of the above-mentioned properties. When the amount of cores ofthe dense amorphous silica is smaller than the above-mentioned range,the tendency of turning into a powder increases and the transparencydecreases. When the amount of shells of the bulky amorphous silica issmaller than the above-mentioned range, the delustering action andantiblocking action decrease, and the abrasion tendency increases.

(10) In the amorphous silica particles of the present invention, it isdesired from the standpoint of preparation that the cores of the denseamorphous silica are gel-method amorphous silica particles and theshells of the bulky amorphous silica are precipitation-method amorphoussilica particles precipitated on the surfaces of the core particles.However, the amorphous silica particles of the present invention are inno way limited to those prepared by the above-mentioned method, as amatter of course.

(11) It is further desired that the cores of the dense amorphous silicahave a BET specific surface area of from 200 to 800 m² /g, and theporous volume of those having porous radii of from 10 to 150 angstromsis from 0.2 to 2 ml/g. Within these ranges, the shells of the bulkyamorphous silica are formed most smoothly and reliably, and variousactions are excellently exhibited in combination.

[Preparation of Amorphous Silica Particles of a Double Structure]

(1) A process for preparing amorphous silica particles of a doublestructure according to the present invention comprises (A) a step ofneutralizing an alkali silicate aqueous solution and a mineral acidaqueous solution under a condition of a pH of from 2 to 10 to preparegel-method amorphous silica, (B) a step of wet-pulverizing the thusformed gel-method amorphous silica, and (C) a step of neutralizing thealkali silicate aqueous solution and the mineral acid aqueous solutionin the presence of the wet-pulverized gel-method amorphous silicaparticles under a condition of a pH of from 5 to 9, so that theprecipitation-method amorphous silica particles are precipitated on thesurfaces of the gel-method amorphous silica particles.

(2) In the step (A), a silica gel of a homogeneous composition isprepared by the neutralization reaction under the above-mentioned pHvalue condition, and is subjected to the wet pulverization in the step(B) to prepare core particles having homogeneous diameters. In the step(C), the neutralization reaction is carried out within theabove-mentioned pH range in the presence of the core particles toreliably form shells of the bulky amorphous silica on the surfaces ofthe core particles, permitting free bulky amorphous silica particles tobe formed very little.

(3) In the process of preparation of the present invention, it isdesired that the gel-method amorphous silica particles are prepared at atemperature of not higher than 50° C. from the standpoint of forminggel-method silica of a homogeneous composition. The silica particlesusing cores prepared under this condition exhibit excellent properties,too.

(4) It is further desired that the gel-method amorphous silica iswet-pulverized such that the volume-based median diameter becomes from0.5 to 5 μm. The amorphous silica particles of the double structureusing, as cores, the gel-method amorphous silica particles that are notwet-pulverized, exhibit poor delustering action and antiblocking actionand conspicuous abrasion tendency.

(5) It is further desired that the precipitation-method amorphous silicaparticles are precipitated at a temperature of from 60 to 100° C. Thismakes it possible to prepare amorphous silica particles having a doublestructure exhibiting excellent delustering action, antiblocking actionand resistance against abrasion while preventing the precipitation offree bulky amorphous silica, maintaining good yield andreproduceability.

(Preparation of Gel-Type Amorphous Silica)

To prepare a slurry of silica hydrogel, an alkali silicate aqueoussolution and an acid aqueous solution are reacted together to prepare asilica hydrogel, first.

The alkali silicate which is a starting material will be sodium silicateor potassium silicate such as water glass or the like that have beenspecified under JIS as industrial products, or alkali silicate obtainedby reacting a reactive silica recovered from a starting clay such asacidic clay with a hydroxide solution of an alkali metal. It is desiredthat the alkali silicate aqueous solution has an SiO₂ concentration offrom 6 to 28% by weight, and that the molar ratio of SiO₂ :M₂ O (M is analkali metal) is, generally, from 2:1 to 4:1 and, particularly, from2.5:1 to 3:1.

Generally, the mineral acid used for the neutralization reaction will behydrochloric acid or sulfuric acid. However, there can be used a mixtureacid thereof. The concentration of the mineral acid aqueous solution isgenerally from 10 to 75% by weight and, particularly, from 20 to 60% byweight.

The neutralization reaction by the contact of the two starting materialsis accomplished by a method in which either one of the two startingmaterials is added into the other solution with stirring, or by a methodin which the two starting material solutions are simultaneouslycontacted under the predetermined conditions. Though there is noparticular limitation, the neutralization temperature is generally nothigher than 50° C., and the pH at the end of the neutralization is from4 to 7.

A hydrosol of silica is formed through the neutralization. After left tostand for more than 30 minutes, in general, the hydrosol is invertedinto a hydrogel.

The SiO₂ concentration in the thus formed hydrogel is usually as low asfrom 5 to 30% by weight. According to the present invention, thehydrogel of the silica is heat-treated in order to adjust the watercontent (to increase the SiO₂ concentration) in addition to adjustingthe porosity of the hydrogel, thereby to obtain a silica hydrogel havingan SiO₂ concentration of not smaller than 5%.

The heat treatment is effected at a temperature of, desirably, from 100to 170° C. in, for example, an autoclave.

The silica hydrogel after the heat treatment is washed with water,coarsely pulverized to have a diameter of generally from 20 to 100 μm,formed into the above-mentioned silica hydrosol slurry having an SiO₂concentration of from 15 to 25% by weight, and is wet-pulverized underhigh-speed shearing.

The wet-pulverization is carried out preferably by using a widely knownfrictional inner plate mill (such as DYNO-MILL manufactured by WILLY A.BACHOFEN AG). It is of course allowable to use any other wet-typepulverizer provided it is capable of executing high-speed shearing. Inthis case, it is important that the slurry temperature does not exceed50° C. from the standpoint of decreasing the aggregation among theparticles.

(Preparation of Amorphous Silica Particles of a Double Structure)

According to the present invention, the alkali silicate aqueous solutionand the mineral acid aqueous solution are neutralized under thecondition of a pH of from 2 to 7 in the presence of gel-method amorphoussilica particles wet-pulverized as described above, so that theprecipitation-method amorphous silica particles are precipitated on thesurfaces of the gel-method amorphous silica particles.

As will be described later, the precipitation-method amorphous silica isformed by reacting the sodium silicate with a mineral acid such ashydrochloric acid, nitric acid or sulfuric acid in a dense alkali metalsalt solution.

In this case, alkali metal salts of an inorganic acid or an organic acidcan be used in a single kind or in a combination of two or more kinds asthe alkali metal salt solution.

When the alkali metal salts are used in such a combination as tofavorably precipitate the amorphous silica giving advantage in cost andforming a table salt as a starting material of silicic acid and acidcomponent, it is allowed to repetitively use these alkali metal salts inthe form of a metal salt solution.

Upon simultaneously pouring sodium silicate and hydrochloric acid into adense saline aqueous solution, the silica is formed due to the doubledecomposition reaction. In the step of this reaction, the pH value mustbe controlled. While the sodium silicate and hydrochloric acid are beingpoured, the pH is maintained to be from 2 to 7 and, particularly, from3.5 to 6. After the simultaneous pouring has finished, the pH ismaintained to be from 2 to 5 to effect the ripening. At the start of thesimultaneous pouring, it is desired that the saline aqueous solution hasa concentration of generally from 5 to 30% by weight and, particularly,from 13 to 18% by weight. At the end of the simultaneous pouring, it isdesired that the silica concentration is from 1 to 20% by weight. It isdesired that the temperature of the reaction during the simultaneouspouring is from 50 to 100° C. and that the reaction based on thesimultaneous pouring terminates in 3 to 20 hours. After the end of thesimultaneous pouring, the ripening should be effected at a temperatureof from 60 to 100° C. for about 30 minutes to about 25 hours. The formedsilica is separated from the mother liquor, washed with water, driedand, as required, is classified to obtain a product.

[Use]

Owing to the above-mentioned properties, the amorphous silica particlesof the present invention are particularly useful as a delustering agentfor coating materials.

The amorphous silica particles of a double structure of the presentinvention have a feature in that the volume concentration of thedelustering pigment (ml/g of the resin) defined by the following formula(1),

    GPV=P/100Pa                                                (1)

wherein P is the amount in grams of the delustered pigment added per 100g of the coating material resin necessary for satisfying a refractiveindex of 50% at 60° gloss, and Pa is a slack apparent density (g/ml) ofthe delustered pigment, is from 0.10 to 0.20. Here, the slack apparentdensity is based on the Japanese Industrial Standard (JIS-K-5101) andcan be measured by Bulk Density Measuring Machine (manufactured byKuramochi Kagaku Kikai Seisakusyo Co.).

The volume concentration (GPV) of the delustered pigment defined by theabove formula (1) represents the volume of the delustering agent thateffectively acts for delutering per the number of grams of the resin inthe coated film that is formed. As will become obvious from Examplesappearing later, in the case of the conventional bulky amorphoussilica-type delustering agent, the volume concentration (GPV) is aslarge as from 0.21 to 0.50 ml/g of the resin. In the case of the presentinvention, on the other hand, the volume concentration (GPV) is from0.10 to 0.20 ml/g of the resin, from which it will be obvious that thevolume of the delustering agent necessary for achieving the samedelustering level is decreased to a very small value compared with thatof the conventional silica-type delustering agent.

The amorphous silica particles used in the present invention can becovered or treated on their surfaces with an inorganic oxide such astitanium oxide, silicon oxide, zirconium oxide, zinc oxide, bariumoxide, magnesium oxide, or calcium oxide, or with a coupling agent ofthe type of silane, titanium or zirconium.

As required, furthermore, the particulate amorphous silica can be coatedwith a metal soap, a resin acid soap, various resins or waxes. Inparticular, the treatment with an olefin resin wax such as polyethylenewax, oxidized polyethylene wax or acid-modified polyethylene wax, orwith a wax such as animal wax, plant wax or mineral wax, is effective inincreasing the delutering effect and in improving resistance againstscars. The treatment of coating can be easily effected by adding anaqueous emulsion of a wax to an amorphous silica cake after it has beenwashed with water. It is desired that the surface is treated with 1 to20 parts by weight of a wax per 100 parts by weight of the amorphoussilica particles.

In the present invention, the above-mentioned porous silica can be usedfor the coating material in combination with other filler or pigment inaddition to being used alone as a delustering agent. The inorganiccomponents that can be used in combination include alumina, attapulgite,kaolin, carbon black, graphite, fine powdery silicic acid, calciumsilicate, diatomaceous earth, magnesium oxide, magnesium hydroxide,aluminum hydroxide, slate powder, sericite, flint, calcium carbonate,talc, feldspar powder, molybdenum disulfide, barite, vermiculite,whiting, mica, agalmatolite clay, gypsum, silicon carbide, zircon, glassbeads, pumice balloon, asbestos, glass fiber, carbon fiber, rock wool,slag wool, boron whisker, stainless steel fiber, titanium white, zincflower, red iron oxide, iron black, yellow iron oxide, zeolite,hydrotalcite, lithium, aluminum, carbonate, titanium yellow, chromiumoxide green, ultramarine, prussian blue, etc.

The amorphous silica particles of the present invention can be blendedin a known coating material so as to be used as a delustered coatingmaterial composition.

Examples of the coating material include widely known coating materialssuch as oil coating material, nitrocellulose coating material, alkydresin coating material, aminoalkyd coating material, vinyl resin coatingmaterial, acrylic resin coating material, epoxy resin coating material,polyester resin coating material and chlorinated rubber-type coatingmaterial, as well as those coating materials containing one or two ormore of rosin, ester gum, pentaresin, cumarone-indene resin, phenolresin, modified phenol resin, malein resin, alkyd resin, amino resin,vinyl resin, petroleum resin, epoxy resin, polyester resin, styreneresin, acrylic resin, silicone resin, rubber-based resin, chlorideresin, urethane resin, polyamide resin, polyimide resin,fluorine-contained resin, and natural or synthetic Japanese lacquer.

Depending upon the use, the coating material may be in any form such assolvent-type coating material, aqueous coating material, ultravioletray-curable coating material or powdery coating material. Among them,the present invention is particularly suited for the solvent-typecoating material and aqueous coating material.

Examples of the organic solvent for the solution-type coating materialinclude aromatic hydrocarbon solvents such as toluene and xylene;aliphatic hydrocarbon solvents such as n-heptane, n-hexane and Isopar;alicyclic hydrocarbon solvents such as cyclohexane and the like; ketonesolvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone,and cyclohexanone; alcohol solvents such as ethanol, propanol, butanol,and diacetone alcohol; ether solvents such as tetrahydrofurane anddioxane; cellosolve solvents such as ethyl cellosolve and butylcellosolve; ester solvents such as ethyl acetate and butyl acetate; andnonprotonic polar solvents such as dimethyl formamide, dimethylacetamide and dimethyl sulfoxide, which can be used in one kind or intwo or more kinds. The concentration of resin components in the startingsolution is generally from 5 to 70% by weight and, particularly, from 10to 60% by weight.

As the aqueous coating material, there can be used a coating material ofthe type of aqueous solution as well as a coating material of theself-emulsified type or of the type emulsified with a surfactant. As theresin for the aqueous coating material, there can be exemplified alkydresin, polyester resin, acrylic resin or epoxy resin solubilized orself-emulsified in an aqueous medium, which may be used in a combinationof two or more kinds. In the self-emulsified resin, the carboxyl groupis neutralized with ammonia or amines, or the amine contained therein isquaternalized to impart self-emulsifying property. There can be furtherused a variety of latex resins. The concentration of the resin componentis generally from to 70% by weight and, particularly, from 20 to 60% byweight.

As the ultraviolet ray (UV)-curable coating material, there can be usedhigh-solid resins such as UV-curable acrylic resin, epoxy resin, vinylurethane resin, acrylic urethane resin or polyester resin in a singlekind or in a combination of two or more kinds.

As the powdery coating material, there can be exemplified thermoplasticresins such as polyamide, polyester, acrylic resin, olefin resin,cellulose derivative, polyether and vinyl chloride resin, as well asresins such as epoxy resin, epoxy/novolak resin, isocyanate orepoxy-curable polyester resin.

According to the present invention, the above-mentioned delusteringagent is added to the above-mentioned coating material so that thevolume concentration (ml/100 g of resin) of the delustered pigmentdefined by the above-mentioned formula (1) is from 0.10 to 0.20. Thismakes it possible to impart a high degree of delustering effect to thesurface of the coated film despite of the blending in a small amount.

The amorphous silica particles of the present invention are furtheruseful as a filler and, particularly, as an antiblocking agent for thethermoplastic resins, thermosetting resins or various rubbers. That is,the amorphous silica particles of the present invention exhibit anequilibrium water content that is suppressed to a relatively low rangeat a relatively high humidity, exhibit low hygroscopic property withoutdeveloping foaming and are, hence, free from developing color due toadsorption relative to other additives, do not lose properties of theadditives, and serves as an excellent blending agent and, particularly,as an antiblocking agent for the resin films. Moreover, the amorphoussilica particles of the present invention do not adsorb water vapor at alarge rate but exhibit sustained property for adsorbing water vapor.Thus, the amorphous silica particles can be used as a hygroscopic agentfor the resins, exhibiting stable and sustained hygroscopic property.That is, the amorphous silica particles do not quickly adsorb water butdoes not release water that is once adsorbed. Accordingly, the amorphoussilica particles are particularly useful as a hygroscopic agent forresins, absorbing moisture in a package or trapping water that permeatesthrough a container.

As the thermoplastic resin that can be blended with the antiblockingagent comprising the amorphous silica particles of the presentinvention, there can be preferably used olefin resins, such as low-,medium- or high-density polyethylene, isotactic polypropylene,syndiotactic polypropylene, or a polypropylene polymer which is acopolymer with these ethylenes or α-olefin, linear low-densitypolyethylene, ethylene/propylene copolymer, polybutene-1,ethylene/butene-1 copolymer, propylene/butene-1 copolymer,ethylene/propylene/butene-1 copolymer, ethylene/vinyl acetate copolymer,lonically crosslinked olefin copolymer (ionomer), and ethylene/acrylicacid ester copolymer, which may be used alone or being blended togetherin two or more kinds. The amorphous silica particles of the presentinvention is useful as an antiblocking agent for the olefin resin filmsprepared by using a metallocene catalyst, and are capable of eliminatingcoloring tendency that could be seen in the conventional antiblockingagents.

The antiblocking agent of the present invention can be further blendedinto any other known resin films such as polyamides like nylon 6, nylon6-6, nylon 6-10, nylon 11 and nylon 12; thermoplastic polyesters likepolyethylene terephthalate and polybutylene terephthalate; as well aspolycarbonate, polysulfone, vinyl chloride rein, vinylidene chlorideresin and vinyl fluoride resin.

When used as an antiblocking agent, it is desired that the amorphoussilica particles are used in an amount of from 0.005 to 10 parts byweight, preferably, from 0.05 to 3.0 parts by weight and, morepreferably, from 0.05 to 1.0 parts by weight per 100 parts by weight ofthe thermoplastic resin.

The amorphous silica particles of the present invention can be blendedas a filler into the above-mentioned thermoplastic resins, variousrubbers or thermosetting resins, as a matter of course.

As the elastomer polymer for rubbers, there can be exemplifiednitrile/butadiene rubber (NBR), styrene/butadiene rubber (SBR),chloroprene rubber (CR), polybutadiene (BR), polyisoprene (IIB), butylrubber, natural rubber, ethylene/propylene rubber (EPR),ethylene/propylene/diene rubber (EPDM), polyurethane, silicone rubberand acrylic rubber; as well as thermoplastic elastomers such asstyrene/butadiene/styrene block copolymer, styrene/isoprene/styreneblock copolymer, hydrogenated styrene/butadiene/styrene block copolymer,and hydrogenated styrene/isoprene/styrene block copolymer.

As the thermosetting resin, there can be exemplified phenol/formaldehyderesin, furan/formaldehyde resin, xylene/formaldehyde resin,ketone/formaldehyde resin, urea formaldehyde resin,melamine/formaldehyde resin, alkyd resin, unsaturated polyester resin,epoxy resin, bismaleimide resin, triallyl cyanulate resin, thermosettingacrylic resin, and silicone resin, which may be used in a combination oftwo or more kinds.

When used as a filler, the amorphous silica particles of the presentinvention can be blended in an amount of from 0.5 to 20 parts by weightand, particularly, from 2 to 10 parts by weight per 100 parts by weightof the thermoplastic resin, thermosetting resin or elastomer.

When used as a hygroscopic filler, the amorphous silica particles of thepresent invention can be blended in an amount of from 0.5 to 20 parts byweight and, particularly, from 2 to 10 parts by weight per 100 parts byweight of the thermoplastic resin, thermosetting resin or elastomer,depending upon the object.

Moreover, the amorphous silica of the present invention can be used forany other uses of an amorphous silica such as coating agent for paper,chromatograph carrier, base agent for cosmetics, coating material forelectronic parts, hygroscopic agent for electronic parts, etc.

EXAMPLES

The present invention will now be described in detail by way ofExamples. The delustering agent and the like agents used in the presentinvention were measured for their properties and were evaluated in amanner as described below.

(1) Particle distribution.

Measured based on the Coulter counter method (model TA-II, manufacturedby Coulter Electronics Co.) by using a 50-μm aperture tube.

(2) Specific surface area, porous volume.

Measured based on the BET method by using Sorptomatic Series 1800manufactured by Carlo Erba Co.

(3) Method of testing delustering effect.

A baked melamine coating material (Amirak #1400 manufactured by KainsaiPaint Co.) was use as a coating agent. A sample of delustering agent wasadded in an amount of 5 parts by weight to 100 parts by weight of thiscoating material and was dispersed therein by using a dispersing device(Disper) at 2500 rpm for 5 minutes. Then, by using a bar coater #14, themixture was applied onto a mirror-coated paper, left to stand at roomtemperature for 20 minutes, and was baked at 140° C. for 20 minutes toprepare a coated plate. Then, 60° gloss (gloss factor %) was measured byusing a digital glossmeter GM-3D manufactured by Murakami ShikisaiGijutsu Kenkyujo Co.

(4) Method of testing precipitation in the resin.

A two-liquid-type polyurethane resin (normally dry Retan Clear No. 2026,manufactured by Kansai Paint Co.) was diluted with a special thinner,and its viscosity was adjusted by using No. 4 Ford Cup. 100 Grams of thethus diluted resin was weighed into a 200-ml beaker, 3.0 g (2.91% byweight) of the sample of the delustering agent was added thereto and wasdispersed therein by using a Disper at 2500 rpm for 10 minutes. Afterthe dispersion, the mixture slurry was left to stand in a 100-mlgraduated colorimetric tube. After 30 days from the start of standingstill, the state of precipitation was photographed. Then, thecolorimetric tube was shook to disperse the slurry again, and theprecipitation was evaluated as follows:

Precipitation: re-dispersion

◯: the formed precipitate dispersed again when slightly shook.

Δ: the formed precipitate dispersed again when strongly shook.

X the formed precipitate did not disperse again despite it is stronglyshook.

(5) Abrasion testing (resistance against scars).

A test piece 1 (paper cut into 3×3 cm) and a test piece 2 (paper cutinto 20×4 cm) were prepared from the coated paper obtained in (3) above,the test piece 2 was secured with its coating being faced upward onto aglass plate, and the test piece 1 was placed thereon in a manner thatthe film faces the test piece 2. At this moment, a weight of 900 g wasplaced on the test piece 1 so that a load of 100 g was exerted per 1cm². Then, the test piece 1 was moved from an end to an end on the testpiece 2, and the abraded surface state of the coated plate was observed.The scars on the coated film were observed by eyes and evaluated afterthe abrasion was effected 5, 15 and 25 times (reciprocally abraded eachtime). Evaluation by eyes:

    ______________________________________                                        Order of evaluation                                                                          Streaks   Spots                                                ______________________________________                                        1              slightly seen                                                                           no                                                     2 clearly seen no                                                             3 clearly seen 30-70% of                                                        the whole                                                                   4 clearly seen more than 70%                                                    of the whole                                                              ______________________________________                                    

Example 1

(First step)

Into a 500-liter stainless steel container were introduced 86 kg ofsodium silicate No. 3 ((JIS K 1408-66(No.3); SiO₂ concentration of 22%))and 253 kg of water, and the mixture was heated up to 35° C. withstirring. After the temperature was raised, sulfuric acid was one-waypoured thereto until the pH was 2.5 thereby to obtain a gel-methodamorphous silica sol. After the addition has been finished, the ripeningwas effected for 5 hours. After the ripening has been finished, thewet-pulverization was effected by using a MYCOLLOIDER.

(Second step)

60 Kilograms of the gel-method amorphous silica sol (SiO₂ content of 3kg) obtained in the above-mentioned first step was weighed andintroduced into a 150-liter stainless steel container and was heated upto 85° C. with stirring. After the temperature was raised, 13.6 kg ofthe sodium silicate No. 3 diluted into a silica concentration of 13% waspoured at a rate of 0.35 L/min and, at the same time, 7% sulfuric acidwas poured for 30 minutes in a manner that the reaction pH was 7.5.After the simultaneous addition has been finished, the ripening waseffected for 30 minutes. After the ripening, sulfuric acid was poured toadjust the pH to be 3.5.

After the pH was adjusted, the filtering and washing were effected toobtain a cake. Thereafter, the filtered cake was dried, pulverized andclassified to obtain amorphous silica particles having a doublestructure. Table 1 shows the properties and the evaluated results of theobtained powder.

Example 2

The procedure was carried out in the same manner as in Example 1 butusing the gel-method amorphous silica sol obtained in the second step ofExample 1 in an amount of 90 kg (SiO₂ content of 4.5 kg). Table 1 showsthe properties and the evaluated results of the obtained powder.

Example 3

The procedure was carried out in the same manner as in Example 1 butusing the gel-method amorphous silica sol obtained in the second step ofExample 1 in an amount of 40 kg (SiO₂ content of 2 kg). Table 1 showsthe properties and the evaluated results of the obtained powder.

Example 4

The procedure was carried out in the same manner as in Example 1 butadjusting the reaction pH to be 6 in precipitating theprecipitation-method amorphous silica particles in the second step inExample 1. Table 1 shows the properties and the evaluated results of theobtained powder.

Example 5

The procedure was carried out in the same manner as in Example 1 butadjusting the reaction temperature to be 70° C. in precipitating theprecipitation-method amorphous silica particles in the second step inExample 1. Table 1 shows the properties and the evaluated results of theobtained powder.

Example 6

A low-molecular polypropylene wax emulsion (UMEX EM-100 manufactured bySanyo Kasei Co.) was added to the cake that has been filtered and washedin the second step in Example 1, in such a manner that the amount of thepolypropylene wax in pure form was 10 parts by weight per 100 parts byweight of the silica, thereby to obtain a surface-treated amorphoussilica. Table 1 shows the properties and evaluated results of theobtained powder.

Example 7

The procedure was conducted in the same manner as in Example 1 butraising the temperature to 45° C. in the first step in Example 1. Table1 shows the properties and evaluated results of the obtained powder.

Example 8

A polyethylene wax emulsion (Hi-Wax 110P manufactured by MitsuiCHEMICALS, Inc.) was added to the cake that has been filtered and washedin the second step in Example 7, in such a manner that the amount of thepolyethylene wax in pure form was 15 parts by weight per 100 parts byweight of the silica, thereby to obtain a surface-treated amorphoussilica. Table 1 shows the properties and evaluated results of theobtained powder.

Example 9

The procedure was conducted in the same manner as in Example 1 butraising the temperature to 25° C. in the first step in Example 1. Table1 shows the properties and evaluated results of the obtained powder.

Example 10

A polyethylene wax emulsion (PARMARIN manufactured by Sanyo Kasei Co.)was added to the cake that has been filtered and washed in the secondstep in Example 9, in such a manner that the amount of the polyethylenewax in pure form was 5 parts by weight per 100 parts by weight of thesilica, thereby to obtain a surface-treated amorphous silica. Table 1shows the properties and evaluated results of the obtained powder.

Comparative Example 1

The procedure was conducted in the same manner as in Example 1 butadjusting the reaction pH to be 4 in precipitating theprecipitation-method amorphous silica particles in the second step inExample 1. Table 2 shows the properties and evaluated results of theobtained powder.

Comparative Example 2

A commercially available gel-type silica (Sylicia #350 manufactured byFuji Silicia Co.) was measured for its properties and was evaluated. Theresults were as shown in Table 2.

Comparative Example 3

A commercially available precipitation-method silica (Mizukasil P-526manufactured by Mizusawa Industrial Chemicals, Ltd.) was measured forits properties and was evaluated. The results were as shown in Table 2.

Comparative Example 4

The silica powders of Comparative Example 2 and Comparative Example 3were mixed together at a weight ratio of 1:1 to measure and evaluate theproperties. The results were as shown in Table 2.

                                      TABLE 1                                     __________________________________________________________________________    Example No.                                                                              1  2  3  4  5  6  7  8  9  10                                      __________________________________________________________________________    Average particle                                                                         2.5                                                                              2.5                                                                              2.5                                                                              2.5                                                                              2.5                                                                              2.5                                                                              2.0                                                                              2.0                                                                              3.0                                                                              3.0                                       diameter (μm)                                                              Specific surface 180 200 175 210 198 170 210 195 170 160                      area (m.sup.2 /g)                                                             Porous volume (ml/g) 0.6 0.7 0.6 0.8 0.7 0.6 0.7 0.7 0.6 0.6                  Delustering property (%) 15 15 16 16 15 15 16 15 15 15                        Precipitating property Δ  Δ  Δ  Δ  Δ                                                ◯  ◯                                                  ◯  Δ  Δ                                                Abrasion testing                         5 times 1 1 1 1 1 1 1 1 1 1                                                   15 times 1 1 1 1 1 1 1 1 1 1                                                  20 times 2 2 2 2 2 2 2 2 2 2                                                __________________________________________________________________________

                  TABLE 2                                                         ______________________________________                                        Comparative Example No.                                                                        1       2       3     4                                      ______________________________________                                        Average particel diameter (μm)                                                              2.5     2.2     3.0   2.7                                      Specific surface area (m.sup.2 /g) 460 250 140 200                            Porous volume (ml/g) 1.0 1.4 0.5 0.9                                          Delustering property 27 25 18 20                                              Precipitating property X Δ  Δ  Δ                            Abrasion testing                                                              5 times 1 1 1 1                                                               15 times 1 3 3 3                                                              25 times 2 4 4 4                                                            ______________________________________                                    

According to the present invention, an alkali silicate aqueous solutionand a mineral acid aqueous solution are neutralized under a condition ofa pH of from 2 to 10 to prepare a gel-method amorphous silica which is,then, wet-pulverized. Then, the alkali silicate aqueous solution and themineral acid aqueous solution are neutralized in the presence of thewet-pulverized silica particles under a condition of a pH of from 5 to 9thereby to prepare an amorphous silica having a double structure of acore of the dense amorphous silica and a shell of the bulky amorphoussilica, having a volume-based median diameter of from 1 to 10 μm andcontaining not more than 10% by volume of fine particles of a diameterof not larger than 0.5 μm.

The amorphous silica particles having a double-wall structure maintainconstant particle diameters not only in the powdery state but also in astate where the particles are blended in a coating material or in aresin, without developing fine particles, exhibiting excellentdelustering action and antiblocking action, exhibiting decreasedabrading property, and exhibiting excellent resistance against scars orabrasion.

What is claimed is:
 1. Amorphous silica particles having a doublestructure, said particles comprising a core of dense gel-methodamorphous silica particle and a shell of bulky precipitation-methodamorphous silica particles, precipitated on the surface of the coreparticle, said double structure particles, having an average particlediameter of from 2 to 5 μm and containing not more than 10% by volume offine particles having a particle diameter of not larger than 0.5 μm. 2.Amorphous silica particles according to claim 1, wherein a BET specificsurface area is from 150 to 400 m² /g and a BET porous volume is from0.2 to 2 ml/g.
 3. Amorphous silica particles according to claim 1,wherein the cores of the dense gel-method amorphous silica and theshells of the bulky precipitation-method amorphous silica are containedat a weight ratio of from 2:8 to 7:3.
 4. Amorphous silica particlesaccording to claim 1, wherein the cores of the dense gel-methodamorphous silica have a BET specific surface area of from 200 to 800 m²/g, and a porous volume with porous radii of from 10 to 150 angstroms offrom 0.2 to 2.0 ml/g.
 5. A delustering agent for coating materialscomprising amorphous silica particles of claim
 1. 6. A delustering agentfor coating materials of claim 5, wherein the surfaces of the amorphoussilica particles are treated with a wax in an amount of from 1 to 20parts by weight per 100 parts by weight of the amorphous silicaparticles.
 7. An antiblocking agent for films comprising amorphoussilica particles of claim
 1. 8. A delustered coating compositioncomprising a coating material comprising a resin and a delusteringeffective amount of the double structure amorphous silica particles asdefined in claim
 1. 9. The delustered coating composition of claim 8,wherein the resin comprises oil, nitrocellulose, alkyd resin, aminoalkyd resin, vinyl resin, acrylic resin, epoxy resin, polyester resin,or chlorinated rubber.
 10. The delustered coating composition of claim8, which is in the form of a solvent-containing coating material, anaqueous coating material, an ultraviolet ray-curable coating material orpowdery coating material.
 11. The delustered coating composition ofclaim 8, which is in the form of a solvent-coating composition oraqueous coating composition and which comprises from about 5 to 70% byweight of resin.
 12. The delustered coating composition of claim 8,wherein the delustering effective amount, of the double structure silicaparticles, in terms of volume concentration, GPV, defined by the formula(1)

    GPV=P/100Pa                                                (1)

where P is the amount in grams of the particles per 100 g of coatingmaterial resin to provide a refractive index of 50% at 60° gloss, and Pais a slack apparent density, in g/ml, of the particles, is from 0.10 to0.20.
 13. A resin composition, comprising at least one resin selectedfrom the group consisting of thermoplastic resins, thermosetting resinsand rubbers, and double structure silica particles according to claim 1.14. The resin composition of claim 13, which is an antiblocking resincomposition, and wherein the resin comprises thermoplastic resin andwherein the amount of the amorphous silica particles is from about 0.005to 10 parts by weight per 100 parts by weight of thermoplastic resin.15. A filled resin composition comprising a resin selected from thegroup consisting of thermoplastic resin, thermosetting resin and rubber,and from about 0.5 to 20 parts by weight of the double structureamorphous silica particles of claim 1, per 100 parts by weight of resin.16. The amorphous silica particles according to claim 1, wherein theweight ratio of cores of dense gel-method silica particles to bulkyprecipitation-method silica particles is from 3:7 to 5:5.